The method for preparing acrylonitrile by ammoxydation has been improved on various points since its success in industrialization and it is said that the method has technically matured. Still, there have been made efforts to improve utility unit consumption and to reduce production cost by achieving production efficiency.
It is well known that the reacted gas obtained from various raw materials contains by-products such as hydrocyanic acid, acetonitrile or aldehyde in addition to acrylonitrile when acrylonitrile is prepared by ammoxydation. The by-products react with acrylonitrile in the presence of unreacted ammonia or react with one another; as a result, high boiling point compounds are produced. The high boiling point compounds cause not only reduction in the yield of acrylonitrile, the target product, but also clogging at various parts in the quenching tower in the downstream processes. Consequently, the by-products must be immediately separated from the reacted gas. Especially, the unreacted ammonia facilitating undesired reactions must be completely removed as soon as possible.
Conventionally, there has been adopted the method for separating unreacted ammonia in the form of a salt and simultaneously removing other impurities and the by-products mentioned above by preliminarily cooling the reacted gas produced in a reactor, immediately sending the cooled reacted gas to a quenching tower, and washing and quenching the reacted gas with water containing acids such as sulfuric acid. As one of the conventional methods excellent in production efficiency, there has been proposed a method employing a multi-story quenching tower to divide a quenching process into two steps or more, which comprises separating most, unreacted ammonia in the form of a salt by contacting the water containing a sufficient amount of sulfuric acid to neutralize the unreacted ammonia and simultaneously condensing not all but a part of the vapor contained in the reacted gas in the first chamber, and condensing most of the residual vapor in the second chamber (U.S. Pat. No. 3,649,179).
This method succeeded in reducing the cost of recovering and treating the salt because of its higher collectability of ammonia and higher concentration of the resultant ammonium salt than the conventional methods. Yet, the collectability of ammonia achieved by this method is not sufficient. Accordingly, problems such as an acrylonitrile loss and trouble caused by clogging due to high boiling point compounds (so called heavies) have still remained.